1. Field of the Invention
The present invention relates to rotary printing cylinders of the type which are fitted with a removable thin flexible impression or printing plate. This type of printing cylinder and printing plate may be used in a wide variety of printing machines and processes common to the industry. More particularly, the present invention relates to a an apparatus and procedure for accurately positioning, securely attaching and selectively releasing the printing plate to the rotary cylinder. The printing plate is initially positively positioned by indexing pins and is secured in operating position on the surface of the cylinder by means of permanent magnets. A novel mechanical linkage alternately advances and retracts the positioning pins and the magnet holding means relative to the cylinder surface to sequentially release and "grab" the printing plate.
2. DESCRIPTION OF THE PRIOR ART
Rotary cylinder printing presses of various types are well-known in the art and provide a continuously rotating impression cylinder for repetitive printing of successive sheets or continuous strip stock. It is common practice to provide a thin flexible impression plate which wraps partially or completely about the printing cylinder and which is readily removable or replaceable. Various means have been devised for releasably holding the flexible printing plate to the cylinder as for instance vacuum, adhesives and mechanical fasteners. Mechanical fasteners such as clamps, wedges or other securing devices have been used to both initially position the plate and to secure it during operation. These approaches are generally cumbersome, complicated and inefficient. Magnetic force has also been utilized to provide the securing or holding force to adhere the flexible printing plate, usually made of thin flexible ferrous material, to the rotary cylinder. The primary problem with utilizing magnetic attraction is the need to initially register or index the flexible plate accurately in the presence of a constant magnetic force. This problem has been alleviated by the use of electromagnetic securing devices located within the cylinder with suitable switching arrangements, the electromagnets may be energized after the flexible ferrous plate has been properly positioned or registered using indexing pins or other mechanical devices. The F. C. Marquardt U.S. Pat. No. 1,531,492 and the J. S. Fleming U.S. Pat. No. 1,939,681 are examples of this type of apparatus. The obvious disadvantage of this approach, of course, is the need for rather complicated switching mechanisms and slip rings and the like for providing electrical current inside the rotating cylinder. Both of these prior art devices are illustrative of the concept of a fixed position magnets and cleats or dogs which initially position the printing plate prior to energization of the electromagnets.
Another common approach is similar to the prior art discussed with the exception that a fixed permanent magnet or magnets are located on the surface or within the rotary cylinder. The permanent magnets tightly hold the thin flexible ferrous plate to the cylinder once it is applied to the surface. In addition, clamp blocks, screw fasteners or indexing pins are utilized to initially position the flexible printing plate and to aid in its adherence to the cylinder. The disadvantage to this approach is the fact that the printing plate can only be removed by prying or forcing it off of the cylinder surface against the constant magnetic force. Examples of this type of device are disclosed in the J. J. Kessler U.S. Pat. No. 2,978,980 and the J. W. Martt U.S. Pat. No. 3,217,645. Here again the magnets are located in fixed position and the aligning pins or other mechanical positioning means are also stationary. The Welch, Jr. U.S. Pat. No. 3,670,646 is illustrative of another approach using magnetic forces wherein magnetic particles are dispersed in the body of the printing plate which is then applied to a ferrous cylinder or other ferrous surface on the cylinder. The Welch, Jr. patent also utilizes the concept of registration pins or dowels for accurately positioning the printing plate on the cylinder. This type of structure also requires that the printing plate be forcibly detached against the attraction of the magnetic particles.
Generally speaking, the use of electromagnetic forces as the securing means is not desirable since the apparatus is expensive, complicated and cumbersome. The operator usually has both hands occupied in manually positioning the cylinder and attempting to initially position the printing plate. His job is only complicated by having to also operate the electromagnetic controls. With the use of permanent magnets, however, it becomes extremely difficult to initially accurately position the plate. The magnetic forces which are strong enough to hold the plate to the rotating cylinder will tend to "grab" the ferrous printing plate the instant it comes into close proximity to the surface of the cylinder.